This invention relates generally to measuring attenuation and crosstalk in communication channels and more particularly to a termination control circuit made of passive components that allow attenuation and crosstalk measurements to be made from a near end of a cable.
Electrical signals are attenuated in cables due to the various capacitive and impedance effects. The amount of signal attenuation varies according to frequency. Thus, to accurately determine the amount of attenuation, measurements must be made at the specific frequency that signals are to be transmitted over the transmission channel.
U.S. Pat. Nos. 3,777,081 and 3,920,935, both to Vierling et al., describe circuits for measuring frequency-dependent attenuation in a telecommunications line. Each circuit in Verling uses a frequency generator and other active components at both a near end and at a far end of the line to measure signal attenuation.
In U.S. Pat. No. 3,777,081, a measurement signal is modulated at the near end of a line and then demodulated at a far end of the line. The demodulated measurement signal is then transmitted back to the near end of the line. Thus, the system in Verling generates a voltage at the far end of the line and receives the measurement signal at the near end of the line.
The circuits described in Verling, however, require multiple signal generators and separate power supplies at both the near end and far end of the line. Thus, the measurement circuitry is rather complex to build and, in turn, too expensive to permanently couple at the end of each trunk in a communication network.
When a signal measurement is taken, measurement circuitry must first be attached to the far end of a first cable under test (CUT). After the first cable is tested, the measurement circuitry must then be detached and then reattached to the far end of the next (CUT). The far end of each cable is typically at a different remote location. Thus, the measurement capacity must be continuously reattached to different (CUT)'s making the measurement process highly time consuming.
Further, active devices used in measurement systems when un-calibrated generate inaccurate cable measurements. For example, if the multiple signal generators in Vierling begin to operate outside an expected frequency range, cable attenuation values will not be accurately recorded. Thus, the active components in Verling must be constantly maintained and calibrated to provide accurate cable measurements.
Crosstalk and particularly near end crosstalk (NEXT) refers to signal interference in a first cable emanating from signal transmissions from cables in close proximity to the first cable. For example, FIG. 1 is a circuit diagram showing a prior art circuit for measuring crosstalk in a transmission line. A first cable 16 comprises a twisted pair of conductors and having a transmitter 18 at a near end and a termination resistor 23 at a far end. Transmitter 18 generates a radio frequency RF signal. The impedance Z.sub.0 of termination resistor 23 matches the characteristic impedance of cable 16.
A second cable 12 comprising another twisted pair of conductors includes a receiver 14 at a near end and a termination resistor 15 at a far end. The impedance Z.sub.1 of termination resistor 15 matches the characteristic impedance of cable 12. Cable 12 is placed in relatively close proximity to cable 16.
Since cables 12 and 16 are placed relatively close together, electronic coupling (.alpha.) is established between the two cables. Cable 12 operating as an antenna receives emissions from cable 16. Thus, by tuning receiver 14 to the transmission frequency of transmitter 18, the amount of crosstalk between cables 16 and 12 can be measured.
To obtain an accurate crosstalk measurement, reflective noise is eliminated in cables 12 and 16. To eliminate signal reflections, the far end of each cable is terminated in its characteristic impedance by coupling termination resistors to the far end of each cable. The termination resistors, however, must then be removed prior to performing other cable tests. For example, the termination resistor would have to be removed prior to performing the signal attenuation measurements shown in Vierling et al. Thus, additional equipment and cable setup time is required each time a new test is performed on a cable.
Accordingly, a need remains for accurately measuring both signal attenuation and crosstalk both quickly and inexpensively.